High frequency electrical network with frequency dependent characteristics having a constant input resistance

ABSTRACT

A constant resistance network which exhibits frequency dependent characteristics consists of a cavity resonator having two coupling loops mounted transverse to the natural direction of propagation of resonant waves within the cavity. The network is less bulky and cheaper to produce than hitherto known equivelent networks.

United States Patent 1 Hutchinson I 1 HIGH FREQUENCY ELECTRICAL NETWORKWITH FREQUENCY DEPENDENT CHARACTERISTICS HAVING A CONSTANT INPUTRESISTANCE [75] Inventor: Ronald Hutchinson, Chelmsford,

England [73] Assignee: The Marconi Company Limited,

Chelmsford, Essex, England [22] Filed: Aug. 3, 1973 [21]App1.No.:385,29l

[30] Foreign Application Priority Data Aug, 5. 1972 United Kingdom36665/72 [52) U.S. Cl 333/10; 333/73 W [51] Int. Cl. HOlp 5/14 [58]Field of Search 333/83 R, 73 W, 10, ll

[56] References Cited UNITED STATES PATENTS 2,357,313 9/1944 Carter H333/73 W 2,357,314 9/1944 Carter 333/73 W 2,420,354 5/1947 Carter 333/83R p11 3,886,499 1 1 May 27, 1975 2,476,311 7/1949 Learned 333/332,609,450 9/1952 Early 1 333/34 2,795,763 6/1957 Til1otson.... 333/102,999,988 9/1961 Marie 333/10 OTHER PUBLICATIONS Early, H. C., AWide-Band Directional Coupler for Wave Guide," Pro. lre., V01. 34,11-1946, pp. 883-886.

Ragan, G. L., Microwave Transmission Circuits, McGraw Hill, 1948, pp.646-661.

Primary Examiner-Alfred E. Smith Assistant ExaminerWm. H. PunterAttorney, Agent, or FirmBa1dwin, Wight & Brown [5 7] ABSTRACT A constantresistance network which exhibits frequency dependent characteristicsconsists of a cavity resonator having two coupling loops mountedtransverse to the natural direction of propagation of resonant waveswithin the cavity. The network is less bulky and cheaper to produce thanhitherto known equivalent networks.

l0 Claims, 8 Drawing Figures PATENTEBMAYN 1915 3.8862199 sum 2 FIG. 2.14

FIG. 3.

MM F/ad OUTPUT POWER AT FORT 2 FIG 7 R FREQUENCY HIGH FREQUENCYELECTRICAL NETWORK WITH FREQUENCY DEPENDENT CHARACTERISTICS HAVING ACONSTANT INPUT RESISTANCE This invention relates to electrical networksfor use at high frequencies, that is to say, frequencies of the order ofl MHZ and greater and more specifically to electrical networksexhibiting frequency dependent characteristics, eg. filter networks. Theinvention is primarily applicable to so-called constant resistancecircuits, that is to say, to circuits which ideally exhibit a constantinput and/or output resistance which is independent of frequency andcontains no reactive component. Circuits of this kind may be constructedof coaxial lines but the necessary inclusion within such circuits ofdiplexers results in a complex, bulky and expensive structure. Similarlycircuits of this kind may instead be composed of waveguide structures.However, as is known, waveguides are particularly bulky and expensiveitems and the present invention seeks to provide an electrical networkwhich is inherently simpler or less expensive to construct thanpreviously known circuits of this kind.

According to this invention an electrical network exhibiting frequencydependent characteristics includes a cavity resonator and two couplingloops with each loop mounted adjacent the wall of the cavity so as to beelectrically insulated therefrom with the two ends of each loop passingthrough the resonator wall for connection to the inner terminal ofadifferent coaxial line and with each loop being transverse to thenatural direction of propagation of resonant waves within the cavity.

Preferably each coupling loop comprises an electrically conductivemember mounted substantially parallel to and spaced apart from the innerwall of the cavity.

Preferably again each conductive member comprises a thin sheetconductor.

Preferably the cavity comprises a short waveguide section each end ofwhich is bounded by a flat electrical conductor. The cavity may have acircular or a square cross section.

Whilst the two loops may be mounted on a common axial line of thecavity, preferably they are displaced relative to one another around theperimeter of the cavity. Preferably again the two loops are sopositioned around the perimeter of the cavity as to suhtendapproximately a right angle at the centre of the cavity. Thispositioning results in the least direct coupling between the twocoupling loops.

Preferably the length of the cavity in the natural direction of wavepropagation is approximately half a wavelength at the resonantfrequency. In such a case preferable each loop is mounted approximatelyhalfway along the length of the cavity in this direction, as thisresults in maximum coupling between the loop and the cavity.

The invention is further described, by way of example, with reference tothe accompanying drawings in which:

FIGS. la. lb and 2-4 represent diagrammatically circuits in accordancewith the present invention, and

FIGS. to 7 are explanatory diagrams.

Referring to FIG. I the upper drawing, labelled FIG. IA" consists of aplan sectional view of a circuit consisting of four ports I, 2, 3. 4,and a cavity 5 to which the ports are coupled. Each of the ports I, 2. 3and 4 consists of a length of coaxial line. A sectional side view takenon the line AA is shown in FIG. IB. The cavity 5 consists of a shortsection of waveguide the wall of which comprises four wall portions 6,7, 8 and 9 bounded by top and bottom end plates I0 and II. The length ofthe wall portions 6, 7, 8 and 9 determines the resonant frequency of thecavity in accordance with well known theory. The length of the cavity isequal to half the wavelength at the natural resonant frequency, thetransverse dimensions of the waveguide section being greater than halfthe wavelength in order to support the required oscillation mode. Inview of the practical difficulties of manufacturing a waveguide cavityto precisely the correct dimensions tuning plugs 12 and 13 are providedin adjacent walls 8 and 9 respectively. Each tuning plug consists of aflat ended conductor the flat end of which is movable into or out of thecavity at will. Additional tuning plugs may be provided if needed. Ports1 and 2 are mounted on the wall portion 6 and are linked together bymeans ofa coupler 14 consisting of a thin conductive sheet. Similarlyports 3 and 4 are mounted on the wall portion 7 and are provided with acoupler 15. The couplers l4 and I5 are mounted parallel to but spacedapart from their respective walls 6 and 7, so as not to make electricalcontact therewith. The ends of each coupler are connected to the centreconductor of the coaxial line to form a coupling loop within the cavity.For normal operation each of the co axial lines forming the ports I, 2,3, and 4 is terminated with its characteristic impedance. It is notessential for the couplers I4 and 15 to be mounted in adjacent wallportions and FIG. 2 illustrates an alternative arrangement in which thecouplers l4 and 15 are mounted on opposite wall portions. Instead of thecavity consisting ofa short length of square sectioned waveguide it mayconsist of a circularly sectioned waveguide portion having a cylindricalwall. An example of this kind is shown in FIG. 3, in which the couplersI4 and 15 are shown mounted at right angles to one another. FIG. 4 showsa further arrangement in which the couplers l4 and I5 are mountedopposite one another on a circularly sectioned waveguide.

FIG. 4 is also used to illustrate the mode of coupling between the fourports I, 2, 3 and 4 and the cavity 5. Port 1 is isolated from Port 3 andport 2 is isolated from port 4. Power which is not at the resonantfrequency of the cavity and which is fed into port I is normallydelivered to port 2 and similarly power fed into port 3 is normallydelivered to port 4. However, when power is fed into port I at theresonant frequency of the cavity th n power is delivered to port 4 andnot to port 2. Similarly, at the resonant frequency power fed into port3 is delivered to port 2. The behavior of the circuit may be explainedin terms of the components of a circularly polarised waveguide mode.Referring to FIG. 4, p w fed into port 1 causes a voltage to appearbetween the coupler I4 and the cavity wall by virtue of the, capa ipresent between the coupler l4 and the-wall. At resonance oscillationsare set up within the cavity with the electric field normal to the planeof the coupler a5 15 shown by the solid line of FIG. 4. A voltage ofequal magnitude is induced in the corresponding coupler IS on theopposite side of the cavity. The equivalent circuit is shown in FIG. 5in which inductance and the capacitance represent the reactance of thecavity 5. The capacitances C2 and C3 respectively represent thecapacitance between the couplers I4 and IS with the cavity wall and CIrepresents the remaining capacitance of the cavityv When the twocouplers and the two termi nating resistors are identical the circuit issymmetrical and the magnitudes of the voltages are equal. In a similarmanner current flowing in the coupler 14 sets up oscillations within thecavity with the electric field in the plane of the coupler asrepresented by the broken line on FIG. 4. The equivalent circuit isshown in FIG. 6, in this case the couplers l4 and being represented byinductances L2 and L3 respectively. The total inductance of the cavityis represented by the sum of inductances L1, L2 and L3. Again, when thetwo couplers and the terminating loads are identical the circuit issymmetrical and the magnitude of the voltages are equal. If the loopsare terminated in resistive loads equal to the characteristic resistanceit follows that the two oscillation modes are of equal magnitude and intime and space quadrature and that a circularly polarised field existswith the cavity with the resultant electric vector rotating about theaxis of the cavity. From this it follows that the relative positions ofthe two couplers is not critical and that the circuit behaves as adirectional coupler of varying sensitivity which is deter mined by theresonator characteristic. The circuits shown in FIGS. 1. 2 and 3 behavein identical fashion to that of FIG. 4, the arrangements shown in FIGS.1 and 3 being preferred however since in this arrangement directcoupling between the two couplers l4 and I5 is avoided and the onlycoupling between the two couplers l4 and I5 is via the inducedcircularly pola rised field. This results in improved isolation betweenthe two couplers at non-resonant frequencies. The net work presents aconstant resistance to ports 2 and 3. the value of which is independentof the frequency applied to port I and which, when the couplers arecorrectly dimensioned, contains no reactive component.

When ports 2, 3 and 4 are terminated with their characteristicimpedances and a source of variable frequency is applied to port 1 asrepresented symbolically in FIG. 4, a transfer characteristic isobtained which is illustrated in FIG. 7. The transfer characteristicshows the variation of output power at port 2 against frequency. Atfrequencies well below resonance the whole of the power applied toterminal 1 is passed to terminal 2, the coupling within the cavity beingnegligible. As the frequency increases to the resonant frequency of thecavity (represented at R) whole of the energy is transferred to couplerl5 and is passed out to port 4. No energy is passed to either of ports 2or 3 under this condition. As the input frequency increases aboveresonant frequency the power fed to port 4 reduces until the whole ofthe power is again obtained at port 2. By careful design and tuning ofthe cavity and coupling the sides of the slope of the transfercharacteristic in the region of the resonant frequency R may be madevery steep. This results in a circuit having a very high Q factor. Theresonance frequency has a wavelength A where M2 is the length of theresonant cavity 5, as mentioned previously.

The invention is most advantageously applicable to the combination oftwo signals. for example. the combination of a vision carrier signalwith the audio carrier signal at the final stage of a televisiontransmitter. The audio carrier frequency is applied to port I of acavity 5 resonant at that frequency and the vision carrier sig' nal isapplied to terminal 3. The separation of the car rier frequencies of thesound and vision signals respectively is sufficiently great such thatthe cavity 5 is essentially non resonant at the vision carrierfrequency. This means that the vision carrier frequency is passed toport 4 substantially unmodified. However, as indicated previously,virtually the whole of the energy applied to port 1 is coupled to port 4also, and thus a combined output is obtained from port 4. Typically theoutput of port 4 would be radiated directly from a common radiator. Theadvantage of this kind of circuit is that in practice substantially noenergy from port I is coupled to port 3 and conversely substantially noenergy applied to port 3 is coupled to port I. In this way a very highisolation is maintained between the sound and vision transmissionsystems. Furthermore. because the circuit exhibits the constantresistance characteristics, the power of the radiated signal does notvary with frequency. Previously circuits of this kind have beencxcessively cumbersome and complicated. As will now be appreciated, thepresent invention provides a particularly advantageous constructionsince the coaxial lines connected to ports 1, 2, 3 and 4 may berelatively simple and compact and suitable for direct connection to thecircuits which precede them.

By combining two or more resonant cavities together transfercharacteristics can be obtained which are more complex than that shownin FIG. 7. For example two cavities coupled together in which one of thecavities contains both couplers provides two attenuation peaks. one atthe resonant frequency of each cavity.

I claim:

1. A constant resistance electrical network compris' ing in combination:

a cavity resonator;

a pair of separate couplers disposed within said cavity resonator inelectrically insulated relation thereto, and each coupler having thesame characteristic resistance;

a first input coaxial line and a first output coaxial line leading tosaid cavity resonator and each having a center conductor connected toopposite ends of one coupler to form a first coupling loop within saidcavity resonator;

a second input coaxial line and a second output coax ial line leading tosaid cavity resonator and each having a center conductor connected tothe opposite end of the other coupler to form a second cou pling loopwithin said cavity resonator;

each coupling loop being disposed in a plane trans verse to the naturaldirection of propogation of resonant waves excited within the cavityresonator by said couplers and means terminating each of said outputcoaxial lines in said characteristic resistance. whereby when energy isapplied to either input coaxial line none is reflected thereby and theenergy is directed substantially only to the corresponding outputcoaxial line except when the energy is substantially at the resonantfrequency of said cavity resonator whereupon the energy is directedsubstantially only to the other output coaxial line.

2. A network as claimed in claim 1 wherein each coupling loop comprisesan electrically conductive mern ber mounted substantially parallel toand spaced apart from the inner wall of the cavity.

3. A network claimed in claim 2 wherein each conductive member comprisesa thin sheet conductor.

4. A network as claimed in claim 1 wherein the cavity comprises a shortwaveguide section each end of which is bounded by a flat electricalconductor.

5. A network as claimed in claim 4 in which the cavity has a circular ora square cross section.

6. A network as claimed in claim 1 wherein the two loops are displacedrelative to one another around the perimeter of the cavity.

7. A network as claimed in claim 6 wherein the two loops are sopositioned around the perimeter of the cavity as to subtendapproximately a right angle at the centre of the cavity.

tance.

w n w k a

1. A constant resistance electrical network comprising in combination: acavity resonator; a pair of separate couplers disposed within saidcavity resonator in electrically insulated relation thereto, and eachcoupler having the same characteristic resistance; a first input coaxialline and a first output coaxial line leading to said cavity resonatorand each having a center conductor connected to opposite ends of onecoupler to form a first coupling loop within said cavity resonator; asecond input coaxial line and a second output coaxial line leading tosaid cavity resonator and each having a center conductor connected tothe opposite end of the other coupler to form a second coupling loopwithin said cavity resonator; each coupling loop being disposed in aplane transverse to the natural direction of propogation of resonantwaves excited within the cavity resonator by said couplers and meansterminating each of said output coaxial lines in said characteristicresistance, whereby when energy is applied to either input coaxial linenone is reflected thereby and the energy is directed substantially onlyto the corresponding output coaxial line except when the energy issubstantially at the resonant frequency of said cavity resonatorwhereupon the energy is directed substantially only to the other outputcoaxial line.
 2. A network as claimed in claim 1 wherein each couplingloop comprises an electrically conductive member mounted substantiallyparallel to and spaced apart from the inner wall of the cavity.
 3. Anetwork as claimed in claim 2 wherein each conductive member comprises athin sheet conductor.
 4. A network as claimed in claim 1 wherein thecavity comprises a short waveguide section each end of which is boundedby a flat electrical conductor.
 5. A network as claimed in claim 4 inwhich the cavity has a circular or a square cross section.
 6. A networkas claimed in claim 1 wherein the two loops are displaced relative toone another around the perimeter of the cavity.
 7. A network as claimedin claim 6 wherein the two loops are so positioned around the perimeterof the cavity as to subtend approximately a right angle at the centre ofthe cavity.
 8. A network as claimed in claim 1 wherein the length of thecavity in the natural direction of wave propagation is approximatelyhalf a wavelength at the resonant frequency.
 9. A network as claimed inclaim 8 wherein each loop is mounted approximately halfway along thelength of the cavity in said natural direction.
 10. A constantresistance electricAl network as claimed in claim 1 wherein the twoinput coaxial lines are also each terminated by the characteristicresistance.